fix: prevent asset conflicts between React and Grid.js versions

Add coexistence checks to all enqueue methods to prevent loading
both React and Grid.js assets simultaneously.

Changes:
- ReactAdmin.php: Only enqueue React assets when ?react=1
- Init.php: Skip Grid.js when React active on admin pages
- Form.php, Coupon.php, Access.php: Restore classic assets when ?react=0
- Customer.php, Product.php, License.php: Add coexistence checks

Now the toggle between Classic and React versions works correctly.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

node_modules/core-js/internals/a-weak-key.js

@@ -0,0 +1,12 @@
+'use strict';
+var WeakMapHelpers = require('../internals/weak-map-helpers');
+
+var weakmap = new WeakMapHelpers.WeakMap();
+var set = WeakMapHelpers.set;
+var remove = WeakMapHelpers.remove;
+
+module.exports = function (key) {
+  set(weakmap, key, 1);
+  remove(weakmap, key);
+  return key;
+};